Ammonium adsorption in superficial North Sea sediments

Adsorption of NH4+ was measured for 15 North Sea sediments having different contents of organic matter and fine particles. Based on a previous study, the stations were grouped in three clusters: I, silty sediments; II, fine sands; and III, medium sands. The sediments span the range of erosive to depositional areas along the main transport route of particulate matter in the North Sea. Contents of organic carbon and fine particles (<6 μm), both providing potential sorption sites for the NH4+, followed this clustering with highest values in silty sediments and lowest in medium sand. Vertical profiles in the sediment of easily exchangeable NH4+ (EEA, extracted with 2m KCI in 2 h) showed lower contents in winter than in summer, with maximum values in the silty sediments (up to 2500 nmol g−1), and lowest in medium sand (<40 nmol g−1), Dimensionless distribution coefficients (ratio EEA:NH4+ dissolved in the pore water, corrected for porosity) were very high (> 100) in the upper 10 mm of some sediments, but were between 1 and 7 below 40 mm depth at all stations. Sorption isotherms were non-linear with steep slopes at NH4+ <40 μmol dm−3. Dimensionless sorption coefficients obtained from the slope of the isotherms were between 0.1 and 12.3 at NH4+, = 1 μmol dm−3, with large variation between stations. At NH4+ = 100 μmol dm−3, sorption coefficients were less variable between 0.2 and 0.9. The data suggest that the high coefficients at low concentrations are caused by sorption onto organic matter, which in turn is associated with clays and silt. Adsorption of NH4+, depends non-linearly on the concentration in the pore water, and the application of constant distribution coefficients to model NH4+, sorption can be a serious over-simplification of reality.